In a synchronous communications network or system, such as a Synchronous Digital Hierarchy (SDH) or SONET transmission network, traffic data signals are transported across the network via transmission paths, or signal paths. A path may comprise a number of sections, each section comprising a transport service between two adjacent nodes of the network. Each network node typically comprises a network element in the form of a synchronous transmission apparatus, such as a multiplexer, a cross-connect or other network equipment. Each path has an associated path overhead (POH) which carries information enabling network elements to monitor the transmission of the traffic signal on the path and to perform, for example, error monitoring or performance monitoring checks.
Monitoring path overhead normally requires relatively complicated signal processing, usually including what is commonly known as pointer processing. Apparatus for performing such signal processing, including pointer processing apparatus, is normally provided at the signal input, or ingress, of a network element. Hence, in a network element the point of ingress of a data signal is normally considered to be the most convenient place at which to monitor the path overhead associated with the data signal.
However, conventional path protection schemes can result in a data signal arriving at a network element on one of two or more paths, and therefore at one of two or more ingresses. In accordance with conventional path protection techniques, the network element selects one or other of these paths as the “active” signal path. Clearly, it would be inappropriate to monitor the path overhead at one ingress if the selected data signal was in fact arriving at another ingress. This is a particular problem in cases where the network element comprises an intermediate node in the path to the signal's destination. In such cases, the data signal which is output from the network element via a particular output, or egress, is selected from one of two or more ingresses, depending on what path protection switching may have taken place. It is therefore preferable to monitor path overhead at the egress to ensure that the path overhead associated with the selected, or active, signal path is monitored.
By way of example, in some circumstances it is important to be able to perform what is known generally as intermediate path performance monitoring (IPPM) whereby the quality/performance of only a portion of a signal path is monitored. Another performance monitoring scheme is known as Tandem Connection Monitoring (TCM) and is defined in International Telecommunications Union Telecommunications Section (ITU-T) standard G.707/Y.1322 dated October 2000 (particularly Appendices C and D) which is hereby incorporated herein by way of reference. Normally, under TCM, when a traffic signal enters a first network element that marks the beginning of a path portion over which intermediate monitoring is required, one or more error, or performance, measurements are made at the ingress where the traffic signal is received. Corresponding error/performance measurements are then taken at the egress of a second network element that marks the end of the path portion. The difference between the error measurements taken at the ingress of the first network element and the egress of the second network element is used as a measure of the performance of the network portion between the first and second network elements.
There is a problem, however, with monitoring path overhead at an egress of a network element. The necessary signal processing apparatus, including pointer interpretation apparatus and normally pointer processing apparatus, is normally only provided at the ingresses of a network element. Since such processing apparatus is relatively expensive in terms of both cost and area, it is considered undesirable also to provide signal processing apparatus for monitoring path overhead at the egresses of a network element.